This invention relates generally to computed tomography (CT) imaging and more particularly, to generating images of a moving object.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In at least one known type of imaging system, commonly known as a computer tomography (CT) system, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast.
In helical scanning, and as explained above, only one view of data is collected at each slice location. To reconstruct an image of a slice, the other view data for the slice is generated based on the data collected for other views. Helical reconstruction algorithms are known, and described, for example, in C. Crawford and K. King, xe2x80x9cComputed Tomography Scanning with Simultaneous Patient Translation,xe2x80x9d Med. Phys. 17(6), November/December 1990.
In order to generate images of a rapidly moving object, such as a heart, known imaging systems have minimized motion artifacts, caused by the movement of the heart, by utilizing a high rotational speed gantry or by incorporating electron beam technology. However, the high speed gantry system significantly increases the force applied to the x-ray source and the detector affecting performance of the system. The electron beam technology requires a very complex design that significantly increases the cost of the scanner. As a result, few system are capable of generating images of a moving heart without generating images containing significant motion artifacts.
To generate images of a moving object, it is desirable to provide an imaging system which gathers segments of projection data of a selected phase of the object so that by combining the segments motion artifacts are minimized. It would also be desirable to provide such a system which generates a cross-sectional image of the entire object for a selected phase of the object.
These and other objects may be attained by a CT imaging system that generates images of an entire object of interest using segments of projection data collected from a plurality of projection angles for a selected phase of the object. In accordance with one embodiment of the present invention, the imaging system includes at least one rotating x-ray source and at least one detector array. A physiological cycle unit, or circuit, is utilized to generate a physiological cycle signal of the object. The cycle signal represents the time period of each cycle of the object including a plurality of phases. To generate an image of the object for a selected phase, an operator selects at least one phase of the object. For each selected phase of the object, at least one segment of projection data is collected during each rotation of each x-ray source.
More specifically, each segment of projection data is generated, or collected, by emitting an x-ray beam toward an x-ray detector array for a determined imaging temporal period for each selected phase during each rotation. Particularly, as each x-ray source is rotated, an x-ray beam is emitted for the determined imaging temporal period. As a result, a segment of projection data is collected via each detector array. Each segment represents a small range of angular positions. By altering a rotational speed of each x-ray source, segments of projection data are collected from different projection angles as each x-ray source is rotated. More particularly, the rotational speed of each x-ray source is altered so that each segment of projection data for each selected phase of the object is collected from a different projection angle, or range of projection angles. By completing a plurality of rotations of each x-ray source, projection data is collected for a projection angle range of (180 degrees plus a fan angle).
To generate an image of the selected phase of the object, the segments of projection data collected from the different projection angles are combined. More specifically, the collected segments for a selected phase of the object are combined into a set of projection data for the selected phase. The projection data set is then used to reconstruct a cross-sectional image of the object for the selected phase.
In alternative embodiments, the imaging system collects segments of projection data for a plurality of phases of the object during each rotation of each x-ray source. More specifically, after selecting a plurality of phases, at least one segment of projection data is collected for each selected phase of the object during each rotation of each x-ray source.
The above described imaging system generates images of a moving object by gathering segments of projection data for a selected phase of the object so that motion artifacts are minimized. In addition, the imaging system generates cross-sectional images of the entire object for each selected phase of the object.